One analytical method used to detect the presence of hydrocarbons in reservoir rock samples is known as open-system pyrolysis. In open-system pyrolysis, a temperature-programmed instrument heats a small amount of a ground rock sample, usually less than 100 mg. The sample is held for three minutes at a starting temperature of 180° C. and then heated to 600° C. at a rate of 25° C. per minute. During this programmed heating, the hydrocarbons driven from the rock sample are recorded as a function of temperature. The pyrolysis equipment is well-known in the art and is available from commercial sources. In the attached drawings, FIG. 1 shows a typical prior art instrument output plot that is known as a pyrogram. A typical analysis results in three peaks. The first is composed of hydrocarbons that can be volatilized, desorbed, and detected while the temperature is held constant for the first 3 minutes of the procedure at or below about 180° C. These are referred to as light volatile hydrocarbons, or light volatiles (LVHC, or LV).
The next phase of pyrolysis consists of a programmed temperature increase from 180° C. to 600° C. that results in two additional distinct peaks. The first of these peaks occurs between 180° C. and about 400° C., and corresponds to thermal desorption of solvent-extractable bitumen, or the light oil fraction. These are called thermally distilled hydrocarbons (TDHC, or TD). The second additional peak (third peak overall) occurs after about 400° C., generally after a minimum in pyrolytic yield is observed. The temperature corresponding to the minimum in pyrolytic yield is referred to as Tmin and extends typically to about 550° C. This peak is due to the pyrolysis (cracking) of heavier hydrocarbons, or asphaltenes. The materials that thermally crack are called thermally cracked hydrocarbons or “pyrolyzables” (TCHC, or TC).
A specialized analytical procedure that is utilized to characterize reservoir rock is know as the Pyrolytic Oil-Productivity Index, or POPI, and is disclosed in U.S. Pat. No. 5,866,814, the disclosure of which is incorporated herein by reference. The POPI and its associated methods as developed in the prior art are most useful when applied to the characterization of oil productive reservoirs. The analysis of core samples is principally concerned with the characterization of hydrocarbon columns to assess oil-water transition zones, free water levels, and the occurrence of tar mats.
The prior art's approach to the use of pyrolysis data has been directed to the assessment of bulk parameters and has not considered the discreet manner in which the data can be exploited to provide more refined characterizing information as has been accomplished herein with the Compositional Modeling method.
The nature of the laboratory analysis of core samples is significantly different than the type of work that is conducted in the field at drilling sites where pyrolysis data is to be used for the assessment of reservoir quality or reservoir injectivity, and tar occurrence, in order to optimize the placement of horizontal power injection wells and hydrocarbon production wells. The known methodology has been found to have limited utility in characterizing reservoir quality in oil-water transition zones.
The application of the prior art POPI methods in the field were not originally expected to be directed to tar detection or, even to a lesser extent, to determining the apparent water saturation (ASw) and other reservoir parameters. Difficulties in the application of the POPI method became apparent during the field work, when significant contamination from drilling additives was observed in the POPI results, as well as a reduction in hydrocarbon yield from cutting samples collected from oil transition zones when these were compared to the pyrograms and POPI results based on core chip samples tested in the laboratory.
The compositional modeling method (the subject invention) overcomes these difficulties through the modeling of the pyrolytic response in terms of end-member components that are present in a reservoir system. It was further adapted to include the step of removing this contamination signal in order to reveal the unaltered characteristics of staining on the cuttings samples. This processing step required the treatment of contamination by two end-members, as contamination from drilling mud could not be modeled as a single component. In order to obtain an adequate match at well drilling sites, the contributions to the pyrolytic signal from diesel contamination and from other mud additives had to be modeled as separate components. Accounting for these additional end-member components made possible the assessment of the unaltered composition of staining on drill cutting samples.
The development of these novel means to quantify the component of the hydrocarbon signal that is attributable to contamination sources led to another novel method for producing useful characterizing information. By providing a plot of the various pyrolysis components alongside well log data it was discovered that the amount of residual contamination that remains after samples of the drill cuttings have been washed is an indication that reservoir fluids have been displaced by the drilling mud that is circulated in the well bore during the drilling operation. Thus, it has been found that in reservoirs where drilling mud cannot invade the pore space, the amount of contamination is either much lower or not present. This observation leads to the conclusion that the residual drilling mud is easily washed from outside surfaces, because it cannot enter the matrix of lower quality rock in the reservoir.
Thus, the method of the invention provides data and means for the assessment of moveable fluids, which is one of the most important parameters in reservoir characterization. In determining this characteristic by pyrolysis, the incursion is detected by quantifying the remnant component in the sample. The theory supporting the methodology is analogous to the principle used in assessing moved hydrocarbons from well logs, where the invasion of mud filtrate into the reservoir rock is detected by differences in the electrical resistivity profile that is observed. The method of analysis of the invention differs from the well log method in the same way that POPI differs in assessing reservoir productivity, i.e., it is based on a direct measurement from a rock sample and therefore provides an independent assessment of fluid moveability.
The method of the invention is equally applicable to the assessment of fluid moveability in the oil-water transition zone as it is in the oil column. This attribute is particularly important for the optimization of the placement of horizontal power water injector wells. The method can provide an assessment of whether tar is present and, if so, in what quantity. It can also be an indicator of whether reservoir fluids can be moved and provide a better understanding of what amount of tar saturation will adversely affect injectivity.
In order to facilitate a full and comprehensive understanding of the novel methods of the invention there follows a listing of definitions of terms that will be used in the detailed description of the invention.
It is therefore a principal present invention to provide improved information and data based on the POPI method as applied to rock samples gathered in filed drilling operations.
The following are related and specific objects of the invention:
a. to provide a method for modeling the composition of organic matter present in rock samples through an iterative process of reconstructing pyrolysis curves from end-member components that are known to exist in an oil reservoir;
b. to provide a method for correcting data for samples that appear to be composed of one principal end-member component by factoring out or subtracting a variable percentage of the interfering end-member component;
c. to provide a method for assessing pyrolytic characteristics of contaminants or other organic components that occur in relatively low concentrations in samples, but nonetheless are components that require identification and quantifying in order to obtain accurate modeling results;
d. to provide a method for adjusting pyrolytic parameters by subtracting contaminants in order to recalculate POPI and other pyrolytic parameters for use in characterizing and modeling of reservoirs that contain interfering OM that is not part of the migrated hydrocarbons, and therefore are not implicitly related to reservoir quality; and
e. to provide a method for assessing hydrocarbon moveability and reservoir injectivity by the determination of drilling mud contamination as a means of assessing the displacement of reservoir fluids.
Definitions
As used herein, the following terms and designations have the meanings indicated
HC—Abbreviation for hydrocarbons, THC is used for Total Hydrocarbons.
LV—Abbreviation for Alight volatile@ components. As used herein, LV refers specifically to the weight in milligrams of HC released per gram of rock at the initial static temperature condition of 180° C. (when the crucible containing the rock sample is inserted into the pyrolytic chamber) prior to the temperature-programmed pyrolysis of the sample.
TD—Abbreviation for “thermally distillable” components. As used herein, TD refers specifically to the weight in milligrams of HC released per gram of rock at a temperature between 180° C. and Tmin(° C.).
TC—Abbreviation for Athermally crackable@ components. As used herein, TC refers specifically to the weight in milligrams of HC released per gram of rock at a temperature between Tmin(° C.) and 600° C.
LV+TD+TC—Represents the total HC released between 180° C. and 600° C. in milligrams of HC released per gram of rock.
POPI—Abbreviation for the Pyrolytic Oil-Productivity Index. The POPI is calculated from the pyrolytic data by the following equation:POPI=ln(LV+TD+TC)×(TD/TC), where ln is the logarithmic value.
Tmin(° C.)—The temperature at which HC volatilization is at a minimum between the temperature of maximum HC volatilization for TD and TC, and is determined where δ(HC)/δ(T)=0, and is negative before and positive after. Alternatively, a temperature of 400° C. can be used for samples where there is no discernable minimum between TD and TC. The latter type of samples generally have very low total HC yield.
Phi(φ)—The average porosity obtained directly from a rock sample or based on measurements by electric logs at a given depth.
So—The saturation of oil (volume/volume on a pore volume basis) either as calculated from electric logs by the Archie equation, or as determined from laboratory data by Dean-Stark analysis, or as by reference to the actual in-reservoir saturation that cannot be measured directly.
Sw—The saturation of water (volume/volume on a pore volume basis), either as calculated from electric logs by the Archie equation; or as determined from laboratory data by Dean-Stark analysis; or as by reference to the actual in-reservoir saturation that cannot be measured directly.
Sxo—The saturation of drilling mud filtrate and representative of the amount of HC displaced by the filtrate, and therefore, movable HC.
THC—Total hydrocarbons.
ASw—The “apparent water saturation” as calculated from pyrolytic data.
Crucible—The stainless steel container in which the sample is pyrolyzed.
End Member (“EM”)—A consistent type of organic matter or hydrocarbon that cannot be identified and distinguished by pyrolytic analysis.
Pyrolytic Characterizing Data (pcd)—Data values measured at a predetermined number of data points, each data point corresponding to a prescribed temperature.
OM—Organic matter, e.g., shale- and coal-like materials.
FID—Flame Ionization Detector